A prior study reported a "supramaximal" dose of GHRH (200 mcg) produces a refractoriness to the GH-releasing effects of a second 200 mcg dose given two hours later in humans, while the GH response to clonidine is not refractory. We replicated that study, in preparation for using that procedure to study the mechanism of the blunted GH response to clonidine in panic anxiety patients.